1. Field of the Invention
The present invention relates to a photomask fabrication method for a semiconductor device, and, more particularly, to an alternating type phase shifting photomask fabrication method.
2. Discussion of the Related Art
FIG. 1 illustrates an alternating type phase shifting photomask according to conventional art. A plurality of opaque patterns 2 are formed on a transparent substrate 1. A phase shifter 3 and a transmissive portion 4 are alternately located between adjacent pairs of the opaque patterns 2. The phase shifter 3 partially covers a corresponding pair of the opaque patterns 2.
FIGS. 2A and 2B illustrate cross-sections taken along lines IIA--IIA and IIB--IIB of FIG. 1, respectively.
As shown in FIG. 2A, the phase shifter 3 partially covers the adjacent opaque patterns 2 to generate a phase-shift along the interface edges of the opaque patterns 2. However, as shown in FIG. 2B, because the phase shifter 3 comes into direct contact with the transparent substrate 1, a phase shift occurs on the upper surface of the transparent substrate 1.
The working principle of the conventional phase shifting photomask will now be described.
When the transparent substrate 1 is placed on an exposure device (not shown), light passes through the transparent substrate 1 (except for portions covered by the opaque patterns 2) and impinges on a wafer. That is, the light that reaches the opaque patterns 2 is blocked by the opaque patterns 2 and does not reach the wafer. The light that passes through the transparent portion 4 of the substrate 1 and the light that passes through the phase shifter 3 reach the wafer, but their respective phases are different from each other. When viewed along an X-axis as shown in FIG. 2A, the light becomes phase-shifted at the interface edges of the opaque patterns 2 because the phase shifter 3 partially covers the opaque patterns 2. Therefore, because light intensity decreases due to this sudden phase-shift, clearer exposures may be obtained with the opaque patterns 2. However, when viewed along a Y-axis as opposed to the X-axis, as shown in FIG. 2B, the transmissive portion 4 of the substrate 1 comes into direct contact with the phase shifter 3, and the phases of the light that passes through the transparent portion and the light that passes through the phase shifter 3 are 180.degree. out of phase, so that where the transmissive portion 4 and the phase shifter 3 come in contact with each other, light intensity is reduced to nearly zero at the interface. Such a radical decrease in light intensity is illustrated in FIG. 3 showing the variation in light intensity versus distance. That is, light intensity measured at the wafer becomes almost zero at the interface portion 3a between the phase shifter 3 and the transparent substrate 1, and it is difficult to obtain a clear pattern along the interface portion 3a.
Referring to U.S. Pat. No. 5,254,418, a phase transition layer (intermediate phase shifter) is provided at the interface portion 3a between the phase shifter 3 and the transmissive portion 4 of the substrate 1, so that the phase becomes gradually shifted, preventing the light intensity from becoming zero (nulled), thus obtaining a clear pattern at the interface portion 3a between the phase shifter 3 and the transparent portion 4.
Referring to FIG. 4 illustrating the method according to U.S. Pat. No. 5,254,418, a plurality of opaque patterns 12 are formed on a transparent substrate 11. A phase shifter 13 is formed on the transparent substrate 11 to partially cover the opaque patterns 12. A phase transition layer 14 is formed entirely covering the transparent substrate 11, the opaque patterns 12 and the phase shifter 13.
The fabrication method of such a composed phase shifting photomask will now be described with reference to FIGS. 5A-5C.
First, as shown in FIG. 5A, a chromium film serving as an opaque layer is deposited on the transparent substrate 11 by using a known sputtering method and an E-beam evaporation method. The chromium film is selectively removed to form a predetermined pattern, obtaining a chromium pattern serving as the opaque pattern 12. An SiO.sub.2 film is formed by using a known CVD (chemical vapor deposition) method entirely covering the transparent substrate 11 and the opaque pattern 12. A positive type photoresist film 20 (which is positive to the E-beam) is formed on the SiO.sub.2 film using a spin-coat method.
As shown in FIG. 5B, a resist pattern 20a corresponding to the phase shifter 13 in FIG. 4 is formed by selectively exposing the resist film 20 using the E-beam.
Using the resist pattern 20a as a mask, the SiO.sub.2 film is etched by an RIE (reactive ion etching) method, forming the phase shifter 13. Then, the resist pattern 20a is eliminated by O.sub.2 plasma etching.
As further shown in FIG. 5C, the phase transition layer 14 serving as a second phase shifting layer is formed on the transparent substrate 11 including the opaque patterns 12 and the phase shifter 13. Here, the second phase transition layer 14 has high viscosity and is formed by an SOG (spin on glass) method having a high reflow characteristic, so that a gradual slope exists along the interface between the phase shifter 13 and the transparent substrate 11. As a result, the light phase does not drastically shift from zero to 180 degrees at the interface region 13a of the phase shifter 13, and the phase shifting occurs gradually so that light intensity does not fall to zero, thereby obtaining a more distinct pattern on the wafer. When the phase transition layer 14 is formed, the variation in light intensity at the interface portion between the phase shifter 13 and the transparent mask substrate 11 is as shown in FIG. 6.
However, when forming a layer in which a phase shift gradually occurs because two phase shifting layers are formed, light transmission decreases. Also, because light reflection and dispersion are greater at the interface portion of the two separate phase shifting layers 13 and 14, the resolution of the mask decreases. In addition, the use of two phase shifting layers increases the size of the phase interface region and decreases mask integration.